Differential Scanning Calorimeter (DSC) With Temperature Controlled Furnace

ABSTRACT

A differential scanning calorimeter apparatus includes reference and sample cells and controlled temperature shields. The temperature of the shields is controlled such that baseline curvature is reduced by eliminating heat flow from the furnaces to their surroundings (quasi adiabatic conditions) and by controlling heat flow through a well defined solid state heat resistance between the furnaces and a temperature controlled heat sink. The temperature of each shield can be controlled independently to reduce differential heat flow over the whole temperature range of the scan, or maintained at a constant temperature for conventional power compensated DSC operation. The temperature/time profile for each shield can be controlled according to actual furnace temperature, obtained from an empty run, or stored in the computer memory and recalled for sample measurements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/931,163, filed Oct. 31, 2007, which application is currently pending,which in turn is a continuation of U.S. patent application Ser. No.11/054,755, filed Feb. 10, 2005 now Pat. No. 7,371,006, issued May 13,2008, and which application claims the benefit under 35 U.S.C. §119(e)of the U.S. Provisional Patent Application Ser. No. 60/521,036 filedFeb. 10, 2004 herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to calorimetric analytical instruments, and moreparticularly to a temperature controlled shield for differentialscanning calorimeter.

BACKGROUND OF THE INVENTION

The differential scanning calorimeter (“DSC”) is an apparatus which,when a sample and a reference substance are placed therein, and thetemperatures of both are varied at a constant rate, detects and analyzesdifferentially a heat flow generated or absorbed by the sample ascompared with the reference substance.

As the sample material goes through various physical changes, such asfusing, crystallization, freezing, oxidation, and the like, itstemperature is affected by the changes in internal energy. Thedifferences in temperature between the sample and the reference arerecorded and, from this data, calculations may be made for determiningthe internal energy changes occurring in the sample. Such information isuseful in evaluating materials such as pharmaceuticals, plastics, filmsand the like.

One type of DSC is power compensation DSC. It is generally structured bya combination of two independent calorimeters for a sample and areference, and both are provided with a resistance temperature sensorand heat flow feedback heater. The average value of temperaturesdetected by both temperature sensors is compared with a temperatureoutput of a temperature programmer which varies at a constant rate. Twocalorimeters are heated up such that both are brought into coincidenceby the heat flow feedback heaters. Also, if a difference is caused intemperature output of the both temperature sensors, both heaters areimmediately increased or decreased in power to return the difference tozero. Thereupon, the difference of power supplied to the both heaters iscontinuously recorded as a differential heat flow signal.

Various power compensated differential scanning calorimeters are knownin the art such as U.S. Pat. No. 6,530,686 (herein incorporated byreference) relating to a DSC having low drift and certain responsecharacteristics. The sample temperature is controlled according to aprogram temperature by a furnace temperature controller, and at the sametime controlled by a detector temperature controller. Also, if atemperature difference occurs, the supply powers to heaters separatelyprovided close to the sample and reference are adjusted such that thetemperature difference is returned to zero by a differential heatcompensating circuit, outputting a difference in supply power as adifferential heat flow.

U.S. Pat. No. 3,263,484 (herein incorporated by reference) relates to amethod of performing an analysis by changing the temperature of a samplematerial in accordance with a desired program by varying the temperatureof an external medium in heat exchanging relationship with the sample.The difference in temperature between the sample and program is measuredand the applied heat is varied to maintain zero temperature differencethere between. The power required to maintain the zero temperaturedifferences is then measured.

The power compensation type differential scanning calorimeter isresponsive and can quickly realize a heat compensation time constant.However, as for the baseline performance, there has been a difficulty inobtaining stability. The main reason of this lies in that the powercompensation type sensor has a large temperature difference fromsurrounding members during measurement with a result that acomparatively large amount of heat leak occurs from the sensor to theoutside, causing a drift factor in the baseline. Moreover, there isoperating difficulty and lag time between cycles due to frost formationon the cells during cool down.

SUMMARY OF THE INVENTION

The present methods and systems provide a differential scanningcalorimeter comprising at least one cell; at least one thermal shieldadjacently positioned to the cell; a heating system capable of heatingthe cell and the thermal shield; and a temperature monitoring devicewhich monitors a temperature differential between the cell and areference. In an embodiment, the thermal shield is a cylinder positionedaround the cell. The thermal shield can comprise a top end, bottom end,and a sidewall which can have a dielectric layer disposed thereupon. Thesidewall can further comprise a perimeter and a groove extending aroundthe perimeter. In an embodiment, the thermal shield further comprises athermocouple disposed upon the sidewall, and the thermocouple cancomprise a thermal resistant wire and a resistive wire. Optionally, thethermal resistant wire is platinum. The thermal shield further cancomprise a temperature sensor disposed upon the sidewall. In someembodiments, the thermal shield further comprises a resistive wiredisposed upon the sidewall for heating the shield. The thermal shieldcan be made of high thermal conducting material which may optionallyinclude one or more of aluminum, copper, ceramic, and silver. In anembodiment, the thermal shield is characterized as quasi adiabatic. Inone embodiment, the sidewall of the thermal shield is between about 0.25mm to about 10 mm thick, and for example, can be about 0.5 mm thick.Optionally, at least one first cover can be disposed on the thermalshield. Optionally, a block is positioned around the thermal shield, andoptionally, a second cover is disposed upon the block. In someembodiments, there is a gap between the cell and the thermal shield. Inone example, the reference is a sample, and optionally, the referencecan be data.

The present teachings also include providing a differential scanningcalorimeter comprising: a sample cell; a reference cell; a first thermalshield adjacently positioned to the sample cell; a second thermal shieldadjacently positioned to the reference cell; a heating system capable ofheating the sample cell, the reference cell, the first thermal shieldand the second thermal shield; and a temperature monitoring device whichmonitors a temperature differential between the sample cell and thereference cell. A first heating device can be coupled to the samplecell, a second heating device can be coupled to the reference cell, athird heating device can be coupled to the first thermal shield, and/ora fourth heating device can be coupled to the second thermal shield.Accordingly, one or more heating devices can be coupled to one or moreof the sample cell, the reference cell, the first thermal shield, andthe second thermal shield. In some embodiments, the differentialscanning calorimeter further comprises a control system capable ofchanging the temperature of the first, second, third, and/or fourthheating devices. Such differential scanning calorimeters can furthercomprise a computer and/or an output. In an embodiment, the firstthermal shield is a cylinder positioned around the sample cell, and thesecond thermal shield is a cylinder positioned around the referencecell.

The present teachings also include methods of monitoring a temperaturedifferential between the sample cell and the reference comprising:providing a differential scanning calorimeter comprising: sample celland a reference; at least one thermal shield adjacently positioned tothe cells; a heating system capable of heating the cell and the thermalshield; and a temperature monitoring device which monitors a temperaturedifferential between the cell and a reference; obtaining signals fromdifferential scanning calorimeter; and calculating temperaturedifferential between the sample cell and the reference using variablesgenerated by signals. In some embodiments, the methods further compriseusing a thermal shield which can be, for example, a cylinder positionedaround the cell.

As used herein the term “adiabatic” refers to a process where a systemdoes not exchange heat with the surroundings during the process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a differential scanning calorimeter (DSC)having at least one thermal shield;

FIG. 2 is a front isometric view of a DSC sub-assembly with atemperature controlled shield such as that shown in FIG. 1;

FIG. 3( a) and FIG. 3( b) are side isometric views of a temperaturecontrolled shield such as that shown in FIG. 2;

FIG. 4( a) is a front isometric view of a DSC measuring system havingtwo sub-assemblies such as shown in FIG. 2. FIG. 4( b) shows measuringsystem of FIG. 4( a) having a metal block. FIG. 4( c) shows measuringsystem of FIG. 4( b) having an additional cover disposed on block;

FIG. 5 is a graph of heat flow of an empty measuring system of FIG. 4(c) at 10 K/min heating from 350 K to 450 K.

DETAILED DESCRIPTION OF THE INVENTION

A schematic diagram of an embodiment of an improved differentialscanning calorimeter (DSC) is shown in FIG. 1. A calorimeter 100 such asPerkinElmer's Diamond DSC may be used to incorporate the presentteachings; however, other models may be used such as PerkinElmer's powercompensated Pyris 1 DSC, PerkinElmer's DSC 7, or the like. Theillustrated DSC has a metal base 2 located in an inner chamber 4 definedby an outer wall 5 which may be a heat shield. Metal base 2 may beconnected to a cooling block (not shown in FIG. 1). Support 6 on base 2holds a reference cell 8 and a sample cell 10, each similar in volumeand mass, and assembled with heating elements 12 and 14. Reference cell8 and sample cell 10 each hold a sample in this case (not shown in FIG.1), however, one of ordinary skill in the art would appreciate that itis possible to leave reference cell 8 empty, and/or that the referencemay be data or a sample which may have known characteristics. Leads 16connect cells 8 and 10 to a power source 18 to supply power to heatingelements 12 and 14 in each of the cells, which may be independentlycontrolled by a processor and/or computer 20. Computer 20 includes aninterface 23 so that the user may provide/input specifications, and amemory 25 for storage, for example, a hard drive or random accessmemory. The illustrated heating elements 12 and 14 are driven by a powersource 18, so cells 8 and 10 may be heated at varied or identical rates,which are controlled by the computer 20.

The rate at which the temperature of the cells changes is referred to asthe scan rate and can be specified by the user through the computerinterface 23. In some embodiments, the scan rate is between 0.01° C. to500° C./min, although other rates can be specified. As shown, a thermaleffect measuring device 22 is connected to a sensor 27 that measures thedifference in temperature between the two cells 8, 10. Sensor 27 may betwo or more separate sensors capable of independent analysis. In someembodiments, the temperature control of the individual shields 24 isrealized by thermal effect measuring device 22 having two independentcontrol loops using a wire such as a platinum wire as thermometer andthe resistive wire as heater. One controller measures the averagetemperature of the two shields and the second controller measures thetemperature difference between both shields (as in the measuring portionof the power compensation DSC). In the illustrated embodiment, theaverage temperatures of the two shields 24 follows a certain function ofthe temperature of the measuring system (the furnaces) and are operatedaccording to the different modes.

Typical sensors include temperature measuring devices such as wirethermometers, thermocouples or semiconducting thermocouples. Thetemperature differential is continuously measured as the cells are beingheated during a scan. The temperature differential data is thentransmitted from thermal effect measuring device 22 to computer 20,where it is saved along with the time of the measurement in the computermemory 25. Output 75 provides data to user such as visual data showinggraphs, alphanumeric symbols, and the like.

The cells 8 and 10 are surrounded by a thermal shield 24 which is shownas a cylinder placed around the cells. During adiabatic operation, thethermal shield 24 aids in reducing heat exchange between the cells andtheir surroundings. The temperature of thermal shield 24 is monitored bythermal effect measuring device 22 and sensors 27 which are mounted onthermal shield 24. Thermal shield 24 is connected to a heating andcooling device 26 which is operated by at least one controller 28. Thesignal to the controller 28 travels to and from computer 20. The outputfrom cell measuring device 22 is sent to computer 20 and used todetermine a signal to transmit to power source 34 and subsequently ontothe controller 28. The temperature information is repeatedly stored inthe computer memory 35 with the temperature differential between cellsand the time of the measurement. For the illustrated embodiments, theoperating range for the calorimeter in terms of the temperature at whichthe cells and shield can be operated is −170° C. to 730° C. The DSC maymeasure the temperature of thermal shield 24 and cells 8 and 10 with atemperature accuracy of ±0.1° C., and adjust the temperature thereofwithin the precision of ±0.1° C. Although not shown in FIG. 1, one ofordinary skill in the art would understand that the DSC may be modifiedwith a wide range of DSC accessories and options from StepScan DSC andautomatic gas switching to cooling devices and the wide variety of knownsample pans.

The illustrated thermal shield 24 is positioned around reference cell 8and sample cells 10. The power to each of these thermal shields 24 isindependently controlled by the output of computer 20. Thermal shield 24is capable of generating heat, and is used to actively reducetemperature differentials between cells 8 and 10.

The heating system is capable of heating cells 8 or 10 and the thermalshield(s) 24 and may comprise the same or different heating elementsdefined by a controller. Where one, two, three, four, or more heatingelements are used, the heating elements may be commonly controlled orindependently controlled by the user.

Through the computer interface 20, the user can select between variousoperational modes, in which thermal shields 24 are not used or variouslevels of use in which thermal shields 24 are used by computer 20 toactively minimize the temperature differential between cells 8 and 10.

Modes of operation of the calorimeter having quasi adiabatic thermalshields 24 include:

Mode A: Both thermal shields 24 (sample shield and reference shield)closely follow the furnace program temperature (adiabatic conditionsregarding heat exchange through the gas).Mode B: Both thermal shields 24 (sample shield and reference shield)closely follow the furnace program temperature (adiabatic conditionsregarding heat exchange through the gas) as in Mode A but a specified,controlled temperature difference between the thermal shields isintroduced to reduce differential heat flow in the measuring system.Mode C: The temperature of each individual thermal shield 24 (sampleshield and reference shield) is controlled separately. To reducebase-line curvature, the temperatures of each shield is adjusted suchthat for all temperatures the differential heat flow is reduced to, forexample, a minimal base-line heat flow. Additionally the heat flowbetween each furnace and the shield is reduced to a value that canrepresent quasi-adiabatic conditions. The temperature function for theshield temperatures may be obtained from an empty run and stored as anarray or as a smooth function in the computer memory. The temperaturefunction can be recalled during the scan to set the shield'stemperatures to minimize base-line heat flow as much as possible.Mode D: Both shields 24 (sample shield and reference shield) can beoperated in a “constant temperature” mode, allowing operation of the DSCin a conventional power compensation mode.Mode E: To increase heat losses to the surrounding, the shields can beset to the temperature of the heat sink before starting the cooling toincrease maximum cooling rate.

Referring now to FIG. 2, DSC sub-assembly with a separate temperaturecontrolled shield is shown. The quasi adiabatic temperature controlledshield 24 is shown encompassing a cell, such as sample cell 10. Shield24 is made of high thermal conducting material such as one or more ofaluminum, copper, ceramic or silver to avoid temperature gradientsacross shield 24. The temperature gradient and the heat losses which areneeded to allow fast cooling are defined by the connections (feet) ofthe shield (not shown in FIG. 2) and the temperature controlled heatsink at the base of the measuring system (not shown in FIG. 2). Theconnections (feet) are constructed of a metal or ceramic material toallow high temperature operation. Shield 24 comprises a thin walled(between 0.25 mm to 10 mm thick, preferably about 0.5 mm) cylinder. Gap54 lies between cell 10 and thermal shield 24. In the illustratedembodiment, the gap 54 has a width between 0.25 mm and 10 mm, and insome embodiments between about 0.5 mm to about 1.5 mm and yet in otherembodiments about 1.0 mm to promote adiabatic processes. First cover 60is shown disposed on top of thermal shield 24.

Referring now to FIG. 3 a thermal shield 24 in the shape of a cylinderis shown having top end 44, bottom end 46 and sidewall 48. First cover60 is shown disposed on top of thermal shield 24. Sidewall 48 is coveredwith a dielectric layer 50, e.g., a glass or an alumina layer, toelectrically isolate wires from the cylinder giving a very shortresponse time. Sidewall 48 has an outer perimeter and is threaded, thusgroove 52 extends around the outer perimeter. Referring to FIG. 3( b),wire 40 is shown placed within groove 52 extending around the perimeterof thermal shield. In some embodiments, wire 40 may be a thermometer orthermocouple disposed upon the sidewall 48 comprising a temperatureresistive wire portion 54 and resistive wire portion 56 as a heater. Insome embodiments, wire 40 is capable of acting as a temperature sensor.Both temperature resistive wire 54 and resistive wire 56 are woundaround the outside of the cylinder covering a large area of sidewall 48to realize a short response time for temperature control. To avoidcontact between the heater and sensor wires, each is contained inseparate parallel grooves and may be fixed in place with hightemperature ceramic glue. Thermal resistant wire 54 is can be made ofplatinum. Although not shown in FIG. 3, in the illustrated embodiment,the platinum wires are connected with gold braze to a platinum connectorribbon providing high temperature operation.

Referring to FIG. 4( a), a DSC measuring system is shown having twosub-assemblies comprising a reference cell 8 and a sample cell 10. Thesubassemblies and circuitry is enclosed in metal base 2. Various leads58 are shown entering the bottom portion of base 2 which are used toprovide signals to and from the computer to the sensors and heaterspositioned in the subassemblies (not shown in FIG. 4( a)). Referencecell 8 and sample cell 10 are shown adjacent one another in the centerof the apparatus. Thermal shields 24 surround reference cell 8 andsample cell 10 and extend up and out of metal base 2. Referring to FIG.4( b), a DSC measuring system having the same features as FIG. 4( a) isshown; however, aluminum block 60 is placed upon metal base 12. Metalblock 60 can be made of aluminum and contains two openings 62 positionedin the center of the block such that the block may be placed on metalbase 2, and snugly surround reference cell 8 and sample cell 10, bothhaving thermal shield 24. FIG. 4( c) shows a second cover 64 placed overblock 60 such that openings 62 become covered.

FIG. 5 shows a graph of heat flow of an empty measuring system at 10K/min heating from 350 K to 450 K using temperature control shieldssurrounding reference cell and sample cell. The effect of the quasiadiabatic temperature controlled shield as operated as described hereinon base-line straightness can be seen in FIG. 5. The temperature controlof the shields improves base-line straightness. In this case, thetemperature of the shields closely followed the temperature of themeasuring system. One of ordinary skill in the art appreciates thatadditional advanced control algorithms may be added to improve base-linestraightness and noise level even further.

Obviously, many modifications may be made without departing from thebasic spirit of the present teachings. Accordingly, it will beappreciated by those skilled in the art that within the scope of theappended claims, the invention may be practiced other than has beenspecifically described herein.

1. A method for manufacturing and operating a differential scanningcalorimeter comprising the steps of: introducing a first cell with acorresponding first thermal shield containing a top end, a bottom end,and a sidewall; incorporating a second cell with corresponding secondthermal shield containing a top end, a bottom end, and a sidewall;linking a processor to a heating system and a temperature monitoringdevice, controlling heat to each of said cells, and monitoring atemperature differential between said first cell and said second cell.2. The method of claim 1 further comprising controlling heat to each ofsaid thermal shields.
 3. The method of claim 2 further comprisingmonitoring a temperature differential between said thermal shields. 4.The method of claim 1 further comprising determining the internal energyof said first cell using said temperature differential calculated fromsaid temperature monitoring device.
 5. The method of claim 4, whereinsaid first cell is a sample.
 6. The method of claim 5, wherein saidsecond cell is a reference cell.
 7. The method of claim 1 furthercomprising disposing a dielectric layer upon the sidewall of saidthermal first thermal shield and said second thermal shield.
 8. Themethod of claim 1 further comprising disposing a thermocouple upon eachof said thermal shields.
 9. The method of claim 1 further comprisingdisposing a temperature sensor upon the sidewall upon each of saidthermal shields.
 10. The method of claim 1 further comprising heatingeach of said thermal shields via a resistive wire disposed upon thesidewall of each said thermal shield.
 11. The method of claim 1 furthercomprising disposing a cover on each of said thermal shields anddisposing a block around each of said thermal shields.
 12. The method ofclaim 11, further comprising disposing a second cover upon said blocks.13. The method of claim 1 further comprising maintaining adiabaticconditions regarding heat exchange through the gas
 14. The method ofclaim 13 further comprising controlling the temperature differencebetween the thermal shields by a differential heat flow in the measuringsystem.
 15. The method of claim 1 further comprising: controlling thetemperature of each thermal shield by adjusting the differential heatflow; incorporating a computer to store a temperature function forshield temperatures; recalling said temperature function to set thetemperature each thermal shield to minimize base-line heat flow.
 16. Themethod of claim 15 further comprising operating said first and secondshields in a constant temperature mode.
 17. The method of claim 16further comprising setting the shields to the temperature of a heat sinkbefore cooling said shields.
 18. A method for manufacturing andoperating a differential scanning calorimeter comprising the steps of:introducing a sample cell and reference cell; positioning a firstthermal shield adjacent to the sample cell; positioning a second thermalshield adjacent to the reference cell and separate from said firstthermal shield; heating said sample cell, said reference cell, saidfirst thermal shield, and said second thermal shield via at least oneheating system; and monitoring the difference in temperature between thesample cell and the reference cell via a temperature monitoring device.19. The method of claim 18 further comprising calculating the internalenergy of said sample cell by subtracting said difference in temperatureof the sample cell and the reference cell from the temperature of thereference cell.
 20. The method of claim 18 further comprising: couplinga first heating device to the sample cell; coupling a second heatingdevice to the reference cell; coupling a third heating device to thefirst thermal shield; coupling a fourth heating device to the secondthermal shield; and linking a control system to said heating devices.